1. Field of the Invention
The present invention relates generally to a method and apparatus for determining a data rate in a cellular mobile communication system using a smart antenna, and in particular, to a method and apparatus for efficiently and stably allocating an appropriate data rate to a mobile station (MS) despite time-variant other-cell interference from neighbor base stations (BSs) arising from the use of smart antennas in the BSs.
2. Description of the Related Art
In a cellular mobile communication system, a BS communicates with a plurality of MSs simultaneously. The Signal-to-Noise Ratio (SNR) strength of a signal received at an MS from the BS varies depending on the location of the MS and an environment in which the MS is placed within a cell area. As the MS is farther from the BS and as more shadowing areas exist in the path between the BS and the MS, the SNR of the received signal becomes lower. With the same transmit power, the BS allocates a relatively high data rate to an MS with a high SNR and a relatively low data rate to an MS with a low SNR, thereby ensuring reception quality at or above an acceptable level.
The cellular mobile communication system can concentrate a transmitted signal in a specific direction based on the location of the MS within the cell by means of a smart antenna. The SNR of a signal received at the MS from the BS is increased by concentrating the transmitted signal into the location of the MS within the cell using the smart antenna. Accordingly, the smart antenna enables a higher data rate for the MS in the same radio channel environment. As to another MS, the smart antenna can also concentrate transmit power into the location of the MS.
Due to the concentration of transmit power from the BS in a specific direction by the smart antenna, the transmit power varies in time and space. Therefore, when neighbor BSs also use smart antennas, interference from the neighbor BSs varies over time.
FIG. 1 is a flowchart illustrating a conventional data rate determining method. Conventionally, a BS determines a data rate for an MS.
Referring to FIG. 1, the BS transmits a common pilot signal all over the coverage area of a cell managed by the BS in step 101. All MSs within the coverage area of the cell measure the SNRs of the received common pilot signal periodically in step 102 and feed back the SNR measurements to the BS periodically in step 103. For the feedback, a Channel Quality Indicator CHannel (CQICH) is additionally specified in the CDMA2000 1xEVDV standards. Similarly, in IEEE 802.16d Orthogonal Frequency Division Multiple Access (OFDMA), a BS allocates CQICHs to MSs so that the MSs can report their SNRs periodically. Accordingly, the BS receives the SNRs from the MSs in step 104.
The BS includes a lookup table of available data rates with respect to SNRs as a system parameter. Referring to the SNR versus the data rate look up table in step 105, the BS determines available data rates for the individual MSs in step 106. The BS then selects MSs to transmit data in the current transmission time interval based on the data rates and notifies the selected MSs of their data rates in step 107. For example, MS 1 receives the transmit rate information in step 108.
An example of the SNR versus data rate lookup table is illustrated in FIG. 4. Referring to FIG. 4, upon receipt of an SNR measurement from an MS, a modulation order and a coding rate corresponding to the SNR measurement are read from the lookup table. Then, a data rate is correspondingly determined.
In CDMA2000 1xEVDV, a Packet Data Control CHannel (PDCCH) delivers information about MSs and data rates for the MSs, simultaneously with a Packet Data CHannel (PDCH). Therefore, the MSs interpret the PDCCH, determine if they will receive data in the current frame, and if they will, determine at what data rates they will receive the data. However, if the MSs will not receive data in the current frame, the BS notifies an MS associated with each subchannel of its data rate by MAP information broadcast within the coverage area of the BS in the IEEE 802.16d OFDMA system.
In the above conventional data rate determining method, the BS reads a data rate for an MS in correspondence with the feedback SNR of a forward pilot signal received at the MS from the SNR versus data rate lookup table.
FIG. 2 illustrates a problem facing the conventional data rate determining method. The conventional problem encountered will be described between two BSs using smart antennas, by way of example.
Referring to FIG. 2, two users 201 and 202 (user 1 and user 2) are located in the cell area of a BS 205 (BS 1). BS 1 transmits data to user 1 in an nth frame and to user 2 in an (n+1)th frame. With a smart antenna, BS 1 concentrates its transmit power in the direction of user 1 in the nth frame, as indicated by reference numeral 203 and in the direction of user 2 in the (n+1)th frame, as indicated by reference numeral 204. If another BS 206 (BS 2) exists around BS 1, the transmit power of BS 1 interferes with a user within the coverage area of BS 2.
In FIG. 2, BS 2 determines a data rate for the (n+1)th frame for a user 207 (user 3) in the nth frame. In the nth frame, the transmit power of BS 1 concentrates toward user 1, not interfering much with BS 2, as indicated by reference numeral 208. Therefore, user 3 feeds back a high SNR measurement of a received pilot signal to BS 2, and BS 2 correspondingly allocates a high data rate to user 3. However, because the transmit power of BS 1 concentrates toward user 2 in the (n+1)th frame, the interference from BS 1 to BS 2 is very strong, as indicated by reference numeral 209. As BS 2 transmits data to user 3 at the high data rate in the (n+1)th frame, the transmission is highly probable to fail due to the strong interference from BS 1. This phenomenon can be explained from Equation (1) below, describing the SNR of a received pilot signal.
                    SNR        =                  S                                    I              other                        +                          N              o                                                          (        1        )            
In Equation (1), S denotes the strength of a pilot signal received at an MS from a serving BS, Iother denotes the strength of pilot signals serving as noise from neighbor BSs, and No denotes Additive White Gaussian Noise (AWGN). The AWGN, No is maintained at a certain level irrespective of time. However, Iother continuously changes depending on the locations of MSs within the neighbor BSs. As a result, the total SNR varies with time, in proportion to the ratio of Iother to No.
As described above, with the use of smart antennas, the transmit power of BSs varies in areas and is concentrated on a different area in each frame. Therefore, the strength of signals received from neighbor cells fluctuates. Because rate decision and data transmission take place in different channel states, a successful reception probability is highly decreased.